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Use small turbines to recharge while driving?

Use small turbines to recharge while driving?

Submitted by silb3r on Tue, 2017-01-03 13:20

I reserved a Model 3 and just rented a Model S for the first time this past weekend. It was my first time even being inside a Tesla and my instinct to reserve a Model 3 was confirmed.

My road trip required quite a few recharging stops and got me thinking: was it ever considered, or how practical would it be, to add several small turbines on the front of the vehicles (below headlamps perhaps) which would catch air and spin during travel and recharge the battery? Obviously they would be flush with the front of the vehicle and probably require a screen or guard to prevent debris contamination. I wonder if perhaps this was something that was considered but ultimately didn't make the final cut and why. I'm also curious whether this would affect wind resistance and cause drag that would ultimately cancel out the amount of additional power it would generate.

My drive took me across vast swathes of open highway so having *something* to generate power during long stretches like that when I didn't brake for hours at a time could enhance range and highway travel.

DTsea |
3 January, 2017

that would DISCHARGE the battery. the battery makes the car move and these devices would increase drag.

Bighorn |
3 January, 2017

see perpetual motion

Tropopause |
3 January, 2017

I think op means wind driven turbines, which would add enormous drag.

Dramsey |
3 January, 2017

"I wonder if perhaps this was something that was considered but ultimately didn't make the final cut and why."

It was never considered because it's a perpetual motion scheme and won't work.

"I'm also curious whether this would affect wind resistance and cause drag that would ultimately cancel out the amount of additional power it would generate."

Precisely. At least you figured it out on your own. Even if you had magic zero-friction turbine bearings, the amount of electricity generated by the turbines would always be less than the energy required to overcome the drag they produced.

EVolution |
3 January, 2017

this would brake you down, which is already what the regenerative braking does

science-isbetter |
3 January, 2017

Denizens of these forums see these kinds of ideas that discard the fundamental principle of conservation of energy. In short, "no." If you can search the forum take a look at posts by:
Bert 19431
Stackgenerator
Bell0040
Michael Wanamaker
At least these, and may others, are looking for something that is impossible.

silb3r |
4 January, 2017

It's not a "perpetual motion scheme," it's a simple question which anyone who understands high-bypass turbofans should be able to grasp. I'm not suggesting this would eliminate the need to charge the battery, but merely contribute to recharging the battery nominally over long stretches of highway driving at significant speed when braking is infrequent.

I envisioned something incorporated into the existing vehicle frame, recessed into the front to face the wind so that oncoming wind spins the turbines and regenerates the battery. Imagine boring two holes into the front grill and adding recessed turbines that face the oncoming wind when you're driving.

Perhaps the right engineer could devise a way to vent the oncoming wind after it passed through the turbine so that the aerodynamic effects are minimal. Something that allows air to pass through a path cut into/through the side view mirror webbing might even work.

reed_lewis |
4 January, 2017

But any time you are using incoming wind to generate power, that means that the car will be slowed down be the drag created by the wind turbine.

The only time this would actually be net positive would be when there is significant headwind. In that case, the headwind could generate some power.

Look at it this way. If you are travelling 60 MPH, you need a certain amount of energy to go that fast. If you take the wind generated by that speed and attempt to convert it back to electricity, the car will slow down faster because the wind would not be able to freely flow by the car. Imagine a 4 X 8 sheet of plywood flat against the direction you are travelling. That would result in significant drag. No matter how 'high-bypass' the turbofans are, they would still introduce drag.

vitor.h.ribeiro |
4 January, 2017

I had the same major idea same time ago - to use the speed of the wind to charge the batteries.
If we could give to the batteries the autonomy of a normal day´s ride, leaving the charge only to the night, it would be a great breakthrought.
So, the car would need to partially charge along the ride.
And i have came back with a solution without creating an obstacle to the car - also i believe will improve the car stability

Dramsey |
4 January, 2017

@silb3r,

"It's not a "perpetual motion scheme," it's a simple question which anyone who understands high-bypass turbofans should be able to grasp."

No. It's a perpetual motion scheme. You're saying that it's possible for a wind-driven turbine connected to a generator to produce more electrical energy than is required to overcome the extra drag on the platform on which the turbine is mounted. An "energy positive" system like this is the textbook definition of perpetual motion.

It's not. Full stop. No engineering nuance; no deep comprehension of turbine design or physics will make it true. It doesn't matter how efficient or inefficient the generators, batteries, and turbines involved are. Any such scheme will always reduce the range of the vehicle.

djharrington |
4 January, 2017

@silb3r, Think of it this way:
If you have a fan running off of a battery, and then place another fan mounted on a generator that is facing the other fan, and that generator is connected back to the battery, will it run forever when you close the circuit?

In order for that answer to be yes, you would have to have several things (none of which are possible):
- perfectly efficient generator
- perfectly efficient motor
- 100% of the kinetic energy in the moving air generated by the motor's fan to be transferred to the generator's fan
- perfectly efficient wires and electronics
- and many others

Sorry, there's no free lunch.

EcLectric |
4 January, 2017

I have a ground-breaking idea! How about a tutorial on conservation of energy that you have to click through in order to get to the forums? You have to click through it and then there will be a test. You have to pass the test in order to post to the forums.

Just to get us started, here is question 1:

The idea of using the 'wind' created by a forward moving vehicle in order to propel that vehicle forward is:

A. Ludicrous
B. Ridiculous
C. Violates the principle of conservation of energy
D. All of the above

DTsea |
4 January, 2017

silb3r,

a high bypass turbofan is driven by a turbine which is powered by burning fuel. it is basically a propeller. it does not generate power.... it converts power to thrust.

using wind to charge the battery is just a less efficient form of regen and cannot, ever, under any circumstances, generate power except on a stationary vehicle pointed into the wind or a car coasting downhill in neutral.

even the headwind case doesnt work since the drive motor has to overcome the headwind plus the drag of the ram air turbine

i am an aeronautical engineer.

Bighorn |
4 January, 2017

A day later and still smacks of perpetual motion.

Silver2K |
4 January, 2017

curious question.

why cant an alternator type unit be encased with the du, gear and inverter to reproduce energy to charge the main battery? you can have 2 in a d model vehicle charging the battery as the car moves. I understand energy would be used to create energy. I wonder if you can reduce the energy used and increase output of the alternators to constantly top of the main pack.

just asking and wondering.

Silver2K |
4 January, 2017

off

soakes |
4 January, 2017

"it's a simple question".
Oh dear. I thought it was a joke!

mike.e |
4 January, 2017

@silb3r

I'm curious, do you understand what a high-bypass turbofan engine becomes a source of when you shut it down in flight?
Hint: It's one of the 4 forces of flight, that starts with a D.

Ross1 |
4 January, 2017

death?

SamO |
5 January, 2017

Stop trying to violate the laws of thermogodamics.

Remnant |
5 January, 2017

@silb3r (OP, January 3, 2017)

<< ... how practical would it be, to add several small turbines on the front of the vehicles (below headlamps perhaps) which would catch air and spin during travel and recharge the battery? >>

You could use such a contraption for regenerative braking (though Tesla is already using regenerative induction in the motors for the same purpose). It would save some tire wear, but would be expensive to install.

A non-braking regenerative device would be suspension, which has been developed by Audi. The regenerative component replaces the telescopic shock absorbers, which makes the suspension smaller and expands the cargo room or provides room for additional gadgetry, such as computer hardware, speakers, motors, etc.

Dramsey |
5 January, 2017

@SilverP85Plus,

"why cant an alternator type unit be encased with the du, gear and inverter to reproduce energy to charge the main battery? you can have 2 in a d model vehicle charging the battery as the car moves. I understand energy would be used to create energy. I wonder if you can reduce the energy used and increase output of the alternators to constantly top of the main pack."

No. You can't.

Silver2K |
5 January, 2017

why though? why can't an alternator work with the drive shafts that extend sideways to the wheels? wouldn't that be the same as an alternator spinning because of the serpentine belt? there has got to be a way. if we can put someone on the moon or a vehicle on mars, we can do this.

Silver2K |
5 January, 2017

keep in mind, the alternator on an ICE will not function without the serpentine belt.

my question is, why can't an alternator type unit be geared to the DU or attached internally to the drive shafts that are always spinning when the vehicle is in motion?

Bighorn |
5 January, 2017

@silver
You can't extract more energy than you put in--it's always a net loss. Substituting regenerative braking for friction braking is obviously a different scenario. You can't put a windmill on top of the car, or anywhere else without increasing the energy of work more than what you can recover. Same with an alternator.

djharrington |
5 January, 2017

Silver, this is basically how the car works already. The motor is acting as the alternator when slowing down. That's your regen. When you intend to slow down, there's energy to be gained (into the battery), and the car already does this. When shock absorbers damp suspension travel, there's energy to be gained. This energy currently gets converted to heat, but traditional shocks could be replaced by energy scavenging units.

In order to make the car better, you have to tap into energy that is currently being wasted by conversion to heat:
Waste: Drag heats the air; Solution: less drag
Waste: Suspension travel heats the shock oil; Solution: Energy scavenging shocks
Waste: Tires heating; Solution: tires with less rolling resistance
Waste: Brake pads and rotors heating up to stop car; Solution: convert kinetic energy of moving car to stored electrical energy in battery (through motor being used as generator/alternator)

The list goes on. I think the low hanging fruit has mostly been picked. As evidenced by the introduction hybrid cars, regen makes a significant impact on economy. I haven't given much thought to calculating the magnitude of energy scavenged from shocks, but I doubt it is very high given the fact it's not been commercialized yet, and the duration of force is quite short in proportion to the overall time spent driving.

Another way of looking at it is that any method of converting one form of energy to another is not perfect. There are always losses. You will always be better off eliminating the source of wasted energy rather than trying to convert the waste to charge a battery. This is why so much effort is placed on aerodynamics.

J.T. |
5 January, 2017

I always liked the big magnet on top of the car and a sheet of metal in front of it so the attraction of the magnet to the metal moves the car.

How can it not work?

Please do not answer that.

Silver2K |
5 January, 2017

@Bh thanks for info.

@dj, "convert kinetic energy of moving car to stored electrical energy in battery (through motor being used as generator/alternator)" exactly what I was trying to convey, but is it enough to keep the car from needing a charge?

Dramsey |
5 January, 2017

@SilverP85Plus,

"why though? why can't an alternator work with the drive shafts that extend sideways to the wheels? wouldn't that be the same as an alternator spinning because of the serpentine belt? there has got to be a way. if we can put someone on the moon or a vehicle on mars, we can do this."

NO. We can't. Because it VIOLATES THE LAWS OF THE UNIVERSE.

I will try to explain again.

" why can't an alternator work with the drive shafts that extend sideways to the wheels? wouldn't that be the same as an alternator spinning because of the serpentine belt?"

It would be the same. But spinning an alternator, however you do it, produces drag. The alternator does not spin freely with no friction. In fact, the more power you pull from an alternator, THE HARDER IT IS TO TURN IT.

This is because an alternator converts physical motion into electrical energy. If you pull more electricity from the alternator, it gets harder to turn (electromagnetic resistance increases). So the alternator imposes a drag on the car.

Alternators are not 100% efficient. So you're always going to get less power from the alternator than the energy you spend turning it.

A good automotive alternator is about 80% efficient. So if the Tesla uses 100 watts of power to spin the alternator, it gets about 80 watts back. You lose another 15% or so charging the battery, so that 80 watts becomes 68 watts of power going into the battery.

You've using 100 watts to generate 68 watts. See the problem?

Bighorn |
5 January, 2017

@silver
Best to stop wondering and pick up an old physics textbook:)

djharrington |
5 January, 2017

The car already does this. We call it regenerative braking. When you need to slow down, that works out nicely. Kinetic energy that would have otherwise been converted to waste heat has been converted to stored electrical energy in a battery.

That conversion is pretty good, but not perfect. I'm sure you're aware that the battery heats up when it's charged. That's waste. There's additional waste going from AC to DC to charge the battery from the motor (regen). It will never be perfect. You'll always regen less than you originally took from the battery in order to get moving. Just a fact of life (or physics).

Buy several and give each of your passengers one then you're set to go anywhere.

Silver2K |
5 January, 2017

I see.
Thanks for explanation. Just picking on some good physics brains to learn something new :)

Thanks!

reed_lewis |
5 January, 2017

As you have learned, anything that produces power attached to the car will cause drag. By introducing drag, you are slowing the car down. But it is impossible to generate the same amount of power that you needed to get to the speed you are travelling.

radami2 |
5 January, 2017

Hey @silbr3, I have a great idea. Draw up the plans and submit it to the patent office. Why, you'll have a response in no time. While you're waiting, build a miniature prototype for proof of concept. That'll show 'em.

DTsea |
6 January, 2017

silverp85plus kudos for your attitude!

nadurse |
6 January, 2017

Over the past two or three holiday dinners, I tried to explain this logic to a family friend who has similar ideas, no luck so far...

dave |
6 January, 2017

Is this thread really happening?

J.T. |
7 January, 2017

@dave Yes, about every 4 months or so.

Bighorn |
7 January, 2017

We forward these along to Mensa.

Frank99 |
7 January, 2017

Many stationary bicycles use a generator for creating the resistance you feel when pedaling. Set the dial to "0", and the output of the generator is open - no current is flowing, and pedalling is easy, essentially set by the friction in the bearings of the pedals/flywheel. Set it to "1", and a small amount of current is allowed to flow (imagine it's used for charging a battery), and the pedaling gets harder. Set it to "9", and all the current that the generator can, well, generate is allowed to flow (maximum battery charging), and only Lance Armstrong is going to be able to pedal for more than a few seconds.

The concept is the same for a turbine - to generate power from it, you have to put power into it. The power comes from the energy available in the air around it. If wind is blowing, and you stick the turbine into the airflow, you create an interesting situation - the turbine is an impediment to the flow of the wind, so there'll be higher pressure in front of the turbine than behind. The turbine exploits this pressure difference, extracting the energy difference between the high-pressure area in front of it and the low-pressure area behind it. If there is minimal load on the turbine (say, no generator), the turbine will spin fast, and in a well-designed turbine there will be minimal pressure difference between front and back. This is why the gigantic power generation windmills get locked down when they're not in use - you don't want the wind to spin the blades too fast and overspeed the blades - they tend to break and go flying around like a several ton machete.

So, let's hook up our generator to the turbine, because we want to generate some electricity. We set the dial to "1" - and now the situation changes a bit. The blades slow down because there's resistance (much like you pedalling on the bike), the air starts piling up in front of the blades (raising pressure). Eventually everything reaches an equilibrium, but at a lower speed than not having the generator. Raise the dial to '9', and you increase the amount of resistance to the blades spinning, air piles up even more (probably to the point of flowing around the turbine rather than through it).

In a car, airflow is generated by driving through the air - it's essentially stationary and you're moving. Efficient cars are designed to slice through the air - the air is moved up and over and around the vehicle in a way designed to leave it as undisturbed as possible. Every bit of air that is moving after you pass is air that now has kinetic energy - and the only way it could have acquired kinetic energy is from your car. That's the source of drag - your car transferring it's kinetic energy into all these bits of air. With less drag, there's less kinetic energy transfer to the air, your car maintains more of its kinetic energy and your battery has to provide less energy to the motors to make up for kinetic energy loss.

Attaching a turbine adds drag. And, because no machine is perfectly efficient, it adds more drag than you can get back from it. If it generates 100 W of energy, it added more than 100 W of drag to the motion of the car. So adding the turbine doesn't reduce battery drain by partially recharging the car - it actually increases battery drain due to the extra drag.

Now, there are possible scenarios where a turbine could help - in the case of a poorly designed car where there are airflows that are going to turn into pure drag anyway. Conceivably you could mount a turbine into that airflow and extract some energy; but your design energy would be better spent fixing the design to eliminate the draggy airflow.

Anybody make it this far?

ColdOne |
7 January, 2017

Clearly the solution is just to mount a huge sail on top of the vehicle, although it may require you to zig zag across the highway at times.

Bighorn |
7 January, 2017

@ColdOne
Tacky:)

Ross1 |
7 January, 2017

Frank, that was all good.
But isnt the question here about the comparison with a solid flat face against letting the air thru the face(= no resistance) but consequently driving a generator?
Are you saying that the mere fact of the generator being present pushes back against the incoming air...oh I dont know how to explain this.
Isnt the pressure against the flat face of the car in the OP's scenario just being replaced by a ducted generator?
Experiment.

Ross1 |
7 January, 2017

Or are you saying that the car can be braked to a stop by extracting power from the genny?

Ross1 |
7 January, 2017

Aka reverse thrust on an aeroplane?

djharrington |
7 January, 2017

Ross, if you could make the car even more aerodynamic by putting a duct from the front of the car to somewhere else (I say "if" because I'm not sure it would actually lower drag), you'd be better off taking those aerodynamic gains than you would by putting a turbine in the tube and extracting that power. For all the reasons that many of us mention above, your generator cannot create more power than the increase in drag it causes. It's always a net loss ... in this universe with its damned physics. Maybe another universe has a physics set that would make this possible.

J.T. |
7 January, 2017

Bh :-)

Frank99 |
7 January, 2017

Ross -
There are certainly complications. If you erect a ducted fan on the top of your car, which of the following has more drag?
1. Cover the opening
2. Remove the cover, and hold the fan stationary
3. Remove the cover, and let the fan spin freely
4. Remove the cover, let the fan spin and attach a generator to extract energy.
Honestly, I don't know. I think the relative drag is dependent on the fan, for example, a two-blade style windmill vs. the dense fan on the front of a modern jet engine. My gut says that for a two-blade style fan, the relative drag would be 1, 4, 3 , 2 (I think).

I think we can agree that in all four situations, the aerodynamic drag of the car is increased, so the motor will need more power to drive down the highway at 80 mph. The question is, can you extract more power out of the turbine than the extra power the motor needs to maintain speed? The unsatisfying answer is "No, because of the second law of thermodynamics". (Google "Ginsberg's theorem" for the working definitions of the three laws of thermodynamics)(No, really, it's quite funny)(Well, if you ever had to take a course that involved the three laws of thermodynamics its funny).

The more satisfying answer is that the amount of power you can get out of a particular turbine at a particular speed is finite, and people with more knowledge than me can calculate that. The drag in Option 4 can be calculated knowing blade shape, generator, etc. If the power is greater than the drag, then a turbine on the roof could be used to gain battery range. There's nothing obvious, nothing in our daily experiences or in our intuition, that says that the drag will always be greater than the power, and must always be greater. This is why people keep coming up with rational-sounding descriptions of machines that get laughed at, and why they get so insistent - it's so danged difficult to do the calculations to prove that it can't work in all possible situations that the only answer that can be given is "No, because the second law of thermodynamics".

Take another look at our turbine getting driven through the air at 80 mph. If the power output from it is greater than the drag, then you could slowly recharge the battery. What happens when the battery is full? You could start supplementing the power sent to the motor - which would make the car go faster, which would generate more excess power, which could be sent to the motor, until you're going very fast. Sounds like a great airplane - tow it down the runway as the excess power starts spinning the propeller faster, and suddenly you're airborne and heading to London without any fuel! I used to try this with small permanent magnet DC motors when I was a kid. If you spun the motor with your fingers, it would rotate for many seconds before stopping. If you shorted the leads of the motor, it wouldn't spin even one rotation after you spun it.

Ross1 |
8 January, 2017

Oh dear, was that Greek or Double Dutch?
If you guys dont know the answer, how come everyone was knocking the OP?

So the experiment:
how would you design one?
A big 4x6 plywood sheet on the roof rack, a hole cut in, housing an alternator..